This proposal deals with the characterization of hematopoietic cells which give rise to myeloid (CFU-DG), erythroid (CFU-DE), and megakaryocytic (CFU-DM) colonies in diffusion chambers (collectively called CFU-D) in mice. We will compare these colony forming cells with the cells which form myeloid (CFU-C), erythroid (BFU-E, CFU-E), and mixed CFU-G/E) colonies in vitro and spleen colonies (CFU-S) in vivo in irradiated mice. Our initial studies will focus on the CFU-DG the most readily detectable of these diffusion chamber stem cells. Based on preliminary studies we hypothesize that the CFU-DG represents an early hematopoietic stem cell which may be more primitive than the CFU-S. We will study the parent-progeny relationship between CFU-DG and other colony forming cells and by utilizing morphological and chromosome markers assess the clonality and differentiative potential of CFU-D. The replicative capacity of CFU-DG will be determined by use of repetitive subculture techiques. We will study the effect of several physiological activities obtained from cell lines on murine hematopoiesis at the level represented by CFU-DG. Comparative studies of the physical characteristics of murine and human CFU-DG suggest that they may represent the same class of stem cells. Demonstration of murine CFU-DG to be a true pluripotent stem would suggest a similar role for the human CFU-DG.